Unlike a small molecule with a definite molecular weight, a polymer can be considered as a mixture of molecules with various molecular weights. The overall molecular weight of a polymer is expressed in two types of averages: number average molecular weight and weight average molecular weight. The number average is the summation of the number fraction of each unique polymer chain times its molecular weight. For each unique chain, the number fraction is the ratio of the number of just those chains to the total number of polymer chains in the sample. The weight average is the summation of the weight fraction of each unique polymer chain times its molecular weight. For each unique chain, the weight fraction is the ratio of the total weight of just those chains to the total weight of the sample. The weight average is higher than the number average unless all the polymer chains have the same molecular weight.